1. Field of the Invention
The present invention relates to energy transfer methods and apparatus. More particularly, the invention relates to providing a sufficiently stable power to a load in an energy transfer system that transfers power across a physical boundary (e.g., from a power supply located external to a body to a load located internal to the body).
2. Related Art
In a variety of scientific, industrial, and medically related applications, it may be desirable to transfer energy or power (energy per unit time) across some type of boundary. For example, one or more devices that require power (e.g., electrical, mechanical, optical, and acoustic devices) may be located within the confines of a closed system, or “body,” in which it may be difficult and/or undesirable to also include a substantial and/or long term source of power. The closed system or body may be delimited by various types of physical boundaries, and the system internal to the boundary may be living or inanimate, may perform a variety of functions, and may have a variety-of operational and physical requirements and/or constraints. In some cases, such requirements and constraints may make the implementation of a substantial and/or long term “internal” power source for internally located devices problematic.
In some closed systems, repeated entry into the system may be undesirable for a variety of reasons. In other closed systems, significant internal power requirements and a limited internal space may prohibit the implementation of a suitably sized internal power source. In yet other systems, contamination and/or security issues may pose particular challenges in implementing an internal power source. For any combination of the foregoing and other reasons, a power source external to the system and some feasible means of transferring power from the external source to one or more internal devices may be preferable in some applications.
One example of a closed system is the human body. In some medically related and scientific applications, a variety of prosthetic and other devices that require power may be surgically implanted within various portions of the body. Some examples of such devices include a synthetic replacement heart, a circulatory blood pump or ventricular assist device (VAD), a cochlear (ear) implant, a pacemaker, and the like. With respect to the human body, issues such as repeated reentry or surgery, internal space limitations, and contamination (e.g., infection) may make the implementation of a suitable internal power source for some of these implanted devices impractical.
Accordingly, in some medical implant applications, “transcutaneous energy transfer” (TET) devices are employed to transfer energy from outside the body to inside the body, to provide power to one or more implanted prostheses or devices from an external power source. One example of a conventional TET device is a transformer that includes a primary winding (or coil) external to the body and a secondary winding internal to the body. Both the primary and secondary windings generally are placed proximate to respective outer and inner layers of a patient's skin; hence, the term “transcutaneous” commonly refers to energy transfer “through the skin.” Energy is transferred from the primary winding to the secondary winding in the form of a magnetic field. The implanted secondary winding converts the transferred energy in the magnetic field to electrical power for the implanted device, which acts as a “load” on the secondary winding.
In general, TET devices differ from conventional power transformers in that power transformers typically include a magnetic core around which the primary and secondary windings are wound, thus fixing the relative positions of the primary and secondary windings. In contrast, the primary and secondary windings of conventional TET devices are not necessarily fixed in position with respect to one another. Accordingly, one issue associated with conventional TET devices is that the power supplied by the secondary winding to a load (e.g., an implanted device) may be quite sensitive to more than nominal or trivial physical displacements of either the primary winding or the secondary winding from an optimum coupling position. The resolution of this issue determines the suitability of the TET technology to a particular type of load.
For example, implanted prostheses or other devices, and particularly an implanted device that performs a life sustaining function, generally must have a consistent source of available power. Without a consistent power source, the implanted device may function erratically or intermittently. Such an erratic or intermittent operation can have undesirable, and in some cases serious life threatening effects on the patient. Accordingly, with TET devices in particular, and other energy transfer systems in general which transfer energy across a boundary, it is desirable to accurately and reliably provide a sufficiently stable power to the load.